The present invention relates to a method of making bumps for use in connecting 2 substrates (for example a printed circuit board and a semiconductor chip) having pads formed thereon in a case of mounting by BGA (ball grid array) or flip chip technique and a semiconductor device made by using the bumps.
One of the technologies for mounting semiconductor chips on a printed circuit board is a BGA (ball grid array) technology in which pads are formed on both of the semiconductor chips and the printed circuit board to be jointed together by way of solder or gold balls. The use of such bumps enables to mount the semiconductor chips on the printed circuit board in higher density than conventional mounting by the use of semiconductor chips having pin-type terminals or by a COB (chip on board) mounting using bonding wires.
On the other hand, the flip chip mounting for directly mounting bare semiconductor chips on a printed circuit board without packaging is gaining popularity in recent years. In the flip chip mounting, pads on the bare chip are also jointed to pads on a printed circuit board by way of bumps.
Known methods of forming bumps for the BGA mounting or the flip chip mounting are a normal bump, a decalcomania transferring bump, ball bump and mesa bump. The normal bump is formed by covering with resist the upper surface of a semiconductor wafer where bumps are not formed and then forming bumps by plating before removing the resist. The decalcomania transferring process is decalcomania transferring bumps at ends of inner leads and the bumps and aluminium electrodes of a semiconductor chip are aligned before being heated and compressed. The ball bump process is to use a wire bonding machine to attach a bump on each pad. Finally, the mesa bump process is integrally forming bumps on ends of inner leads.
Among these 4 bump forming processes, the decalcomania transferring and mesa bump processes require inner leads which are not suitable for BGA or flip chip mounting. Also, the ball bump process requires to attach bumps on the pads sequentially. This process is time consuming if the number of bumps increases. On the other hand, bumps are formed by plating process in the normal bump process and tend to vary in size and shape.
In light of the above disadvantages of conventional bump forming processes, the object of the present invention is to provide a method of forming bumps of desired size and shape without requiring complex steps and also to provide a semiconductor device using such bumps.
The method of forming bumps in the present invention comprises the steps of covering the upper surface of a first substrate with a resist except pad formed areas, spraying electrically conductive material on the surface covered with the resist of the first substrate that is oriented downwardly, and forming semispherical bumps at the bump forming areas. By orienting the pad formed areas downwardly and spraying electrically conductive material upwardly onto the pad formed areas, gravity helps to form semispherical bumps at the pad formed areas, thereby avoiding complex steps for forming the bumps.
Also, the method of forming bumps according to the present invention comprises the steps of forming printed pattern of desired thickness by screen printing about pads on a first substrate, and forming substantially semispherical bumps by utilizing surface tension to deform the printed pattern. After forming printed pattern by screen printing about the pads on the first substrate, the printed pattern deforms spherically by surface tension, thereby forming semispherical bumps without need for any special machines, etc.
Another method of forming bumps according to the present invention comprises the steps of screen printing on the upper surface of a first substrate to form a printed pattern of a desired thickness about pads on the first substrate, and keeping the first substrate with the surface having the printed pattern facing downwardly for a sufficient time to form the printed pattern in a semispherical shape. Since the printed pattern is formed about the pads on the first substrate by screen printing and the surface having the printed pattern thereon is faced downwardly for a predetermined time, gravity helps to form spherical bumps very easily.